On Wed, Aug 7, 2024 at 6:39 PM Yifan Cai <yc25c...@gmail.com> wrote:
With WatcherService, when events are missed (which is to be
expected), you will still need to list the files. It seems to
me that WatcherService doesn't offer significant benefits in
this case.
Yeah I think we leave it out eventually.
Regarding listing directory with a refresh flag, my concern
is the potential for abuse. End-users might/could always
refresh before listing, which could undermine the purpose of
caching. Perhaps Jeremiah can provide more insight on this.
Well, by default, it would not be refreshed every single time.
You would need to opt-in into that. If there is a shop which has
users with a direct access to the disk of Cassandra nodes and
they are removing data manually, I do not know what to say, what
is nodetool clearsnapshot and jmx methods good for then? I do not
think we can prevent people from shooting into their feet if they
are absolutely willing to do that.
If they want to refresh that every time, that would be equal to
the current behavior. It would be at most as "bad" as it is now.
IMO, caching is best handled internally. I have a few
UX-related questions:
- Is it valid or acceptable to return stale data? If so,
end-users have to do some form of validation before consuming
each snapshot to account for potential deletions.
answer below
- Even if listsnapshot returns the most recent data, is it
possible that some of the directories get deleted when
end-users are accessing them? I think it is true. It, then,
enforces end-users to do some validation first, similar to
handling stale data.
I think that what you were trying to say is that when at time T0
somebody lists snapshots and at T1 somebody removes a snapshot
manually then the list of snapshots is not actual anymore? Sure.
That is a thing. This is how it currently works.
Now, we want to cache them, so if you clear a snapshot which is
not physically there because somebody removed it manually, that
should be a no-op, it will be just removed from the internal
tracker. So, if it is at disk and in cache and you clear it, then
all is fine. It is fine too if it is not on disk anymore and you
clear it, then it is just removed internally. It would fail only
in case you want to remove a snapshot which is not cached,
regardless whether it is on disk or not. The deletion of
non-existing snapshot ends up with a failure, nothing should be
changed in that regard, this is the current behavior too.
I want to say that I did not write it completely correctly at the
very beginning of this thread. Currently, we are caching only
_expiring_ snapshots, because we need to know what is their time
of removal so we act on it later. _normal_ snapshots are _not_
cached _yet_. I spent so much time with 18111 that I live in a
reality where it is already in, I forgot this is not actually in
place yet, we are very close to that.
OK thank you all for your insights, I will NOT use inotify.
Just my 2 cents.
- Yifan
On Wed, Aug 7, 2024 at 6:03 AM Štefan Miklošovič
<smikloso...@apache.org> wrote:
Yes, for example as reported here
https://issues.apache.org/jira/browse/CASSANDRA-13338
People who are charting this in monitoring dashboards
might also hit this.
On Wed, Aug 7, 2024 at 2:59 PM J. D. Jordan
<jeremiah.jor...@gmail.com> wrote:
If you have a lot of snapshots and have for example a
metric monitoring them and their sizes, if you don’t
cache it, creating the metric can cause performance
degradation. We added the cache because we saw this
happen to databases more than once.
> On Aug 7, 2024, at 7:54 AM, Josh McKenzie
<jmcken...@apache.org> wrote:
>
>
>>
>> Snapshot metadata are currently stored in memory /
they are cached so we do not need to go to disk every
single time we want to list them, the more snapshots
we have, the worse it is.
> Are we enumerating our snapshots somewhere on the
hot path, or is this performance concern misplaced?
>
>> On Wed, Aug 7, 2024, at 7:44 AM, Štefan Miklošovič
wrote:
>> Snapshot metadata are currently stored in memory /
they are cached so we do not need to go to disk every
single time we want to list them, the more snapshots
we have, the worse it is.
>>
>> When a snapshot is _manually_ removed from disk,
not from nodetool clearsnapshot, just by rm -rf on a
respective snapshot directory, then such snapshot
will be still visible in nodetool listsnapshots.
Manual removal of a snapshot might be done e.g. by
accident or by some "impatient" operator who just
goes to disk and removes it there instead of using
nodetool or respective JMX method.
>>
>> To improve UX here, what I came up with is that we
might use Java's WatchService where each snapshot dir
would be registered. WatchService is part of Java, it
uses inotify subsystem which is what Linux kernel
offers. The result of doing it is that once a
snapshot dir is registered to be watched and when it
is removed then we are notified about that via
inotify / WatchService so we can react on it and
remove the in-memory representation of that so it
will not be visible in the output anymore.
>>
>> While this works, there are some questions / concerns
>>
>> 1) What do people think about inotify in general?
I tested this on 10k snapshots and it seems to work
just fine, nevertheless there is in general no strong
guarantee that every single event will come through,
there is also a family of kernel parameters around
this where more tuning can be done etc. It is also
questionable how this will behave on other systems
from Linux (Mac etc). While JRE running on different
platforms also implements this, I am not completely
sure these implementations are quality-wise the same
as for Linux etc. There is a history of
not-so-quality implementations for other systems
(events not coming through on Macs etc) and while I
think we are safe on Linux, I am not sure we want to
go with this elsewhere.
>>
>> 2) inotify brings more entropy into the codebase,
it is another thing we need to take care of etc
(however, it is all concentrated in one class and
pretty much "isolated" from everything else)
>>
>> I made this feature optional and it is turned off
by default so people need to explicitly opt-in into
this so we are not forcing it on anybody.
>>
>> If we do not want to go with inotify, another
option would be to have a background thread which
would periodically check if a manifest exists on a
disk, if it does not, then a snapshot does not
either. While this works, what I do not like about
this is that the primary reason we moved it to memory
was to bypass IO as much as possible yet here we
would introduce another check which would go to disk,
and this would be done periodically, which beats the
whole purpose. If an operator lists snapshots once a
week and there is a background check running every 10
minutes (for example), then the cummulative number of
IO operations migth be bigger than us just doing
nothing at all. For this reason, if we do not want to
go with inotify, I would also not implement any
automatic background check. Instead of that, there
would be SnapshotManagerMbean#refresh() method which
would just forcibly reload all snapshots from
scratch. I think that manual deletion of snapshots is
not something a user would do regularly, snapshots
are meant to be removed via nodetool or JMX. If
manual removal ever happens then in order to make it
synchronized again, the refreshing of these snapshots
would be required. There might be an additional flag
in nodetool listsnapshots, once specified, refreshing
would be done, otherwise not.
>>
>> How does this all sound to people?
>>
>> Regards
>
